This invention is in the field of zeolite and silicoaluminophosphate (SAPO) membranes, in particular SAPO-34 membranes, prepared using organic templating agents. The invention provides improved supported SAPO membranes as well as methods for making and using such membranes.
In the past two decades, extensive research has been devoted to SAPO and other zeolite membranes because they have higher thermal and chemical properties compared with those of polymer membranes. Many types of zeolite membranes have been studied such as MFI, LTA, MOR, and FAU-type membranes. (See Sano et al, Chem. Lett. 12 (1992), p. 2413; Bakker et al, J. Membr. Sci. 117 (1996), p. 57; Lai et al, Ind. Eng. Chem. Res. 37 (1998), p. 4275; Liu et al, Chem. Commun. (2000), p. 1889; Bernal et al, Catal. Today 67 (2001), p. 101; Hedlund et al, J. Membr. Sci. 52 (2002), p. 179; Lai et al, Science 300 (2003), p. 456; Li et al, Ind. Eng. Chem. Res. 40 (2001), p. 4577; Kita et al, J. Mater. Sci. Lett. 14 (1995), p. 206; Kondo et al, J. Membr. Sci. 133 (1997), p. 133; Jafar and Budd, Microporous Mater. 12 (1997), p. 305; Aoki et al, J. Membr. Sci 141 (1998), p. 197; Kumakiri et al, Ind. Eng. Chem. Res. 38 (1999), p. 4689; Okamoto et al, Ind. Eng. Chem. Res. 40 (2001), p. 163; Morigami et al, Sep. Purif. Technol. 25 (2001), p. 251; Van den Berg et al, J. Membr. Sci. 224 (2003), p. 29; Pina et al, J. Membr. Sci. 244 (2004), p. 141; Huang et al, J. Membr. Sci. 245 (2004), p. 41; Sato et al, J. Membr. Sci. 301 (2007), p. 151; Nishiyama et al, J. Chem. Soc. Chem. Commun. (1995), p. 1967; Tavolaro et al, J. Mater. Chem. 10 (2000), p. 1131; Zhang et al, J. Membr. Sci. 210 (2002), p. 361; Li et al, Microporous Mater. 62 (2003), p. 211; Nikolakis et al, J. Membr. Sci. 184 (2001), p. 209; Kita et al, Sep. Purif. Technol. 25 (2001), p. 261; Matsukata and Kikuchi, Bull. Chem. Soc. Jpn. 70 (1997), p. 2341; Caro et al, Microporous Mesoporous Mater. 38 (2000), p. 3; and Bowen et al, J. Membr. Sci. 245 (2004), p. 1).
SAPO membranes have great potential in chemical and petrochemical industries for large scale separations, such as natural gas sweetening and carbon dioxide (CO2) sequestration. For these applications, important parameters are the permeance (the degree to which the membrane admits a flow of a particular gas through the membrane) and the separation selectivity provided by the membrane. For two gas components i and j, a separation selectivity Si/j greater than one implies that the membrane is selectively permeable to component i. If a feedstream containing both components is applied to one side of the membrane, the permeate stream exiting the other side of the membrane will be enriched in component i and depleted in component j. The greater the separation selectivity, the greater the enrichment of the permeate stream in component i.
Carbon dioxide/methane (CO2/CH4) separation is important for natural gas processing because CO2, which is a contaminant in natural gas wells, decreases the energy content of the gas, and is acidic and corrosive in the presence of water. It has been reported that SAPO-34 membranes have high CO2/CH4 separation selectivities, but that the selectivities decrease as the feed pressure increases because at higher feed pressures a larger fraction of the gas flow is through defects in the membrane and CO2 loading is closer to saturation loading than CH4 (Li et al., Ind. Eng. Chem. Res., 44 (2005) p. 3220; Carreon et al., J. Am. Chem. Soc., 130 (2008) p. 5412). Because natural gas wells are at high pressures, the gas separation also needs to be done at high pressure and CO2 needs to be removed while keeping CH4 at high pressure. Accordingly, it is desirable to produce SAPO and other zeolite membranes having high permeance and separation selectivities, particularly CO2 permeance and CO2/CH4 separation selectivities, at high pressure.
SAPO crystals can be synthesized by hydrothermal crystallization from a reaction mixture containing reactive sources of silica, alumina, and phosphate, and an organic templating agent. Lok et al. (U.S. Pat. No. 4,440,871) report gel compositions and procedures for forming several types of SAPO crystals, including SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-37, SAPO-40, SAPO 41, SAPO-42, and SAPO-44 crystals. Similarly, Prakash and Unnikrishnan report gel compositions and procedures for forming SAPO-34 crystals. (Prakash and Unnikrishnan, J. Chem. Sc. Faraday Trans., 90(15) (1994), p. 2291-2296). In several of Prakash and Unnikrishnan's reported procedures, the gel was aged for 24 hours at 27° C. (300 K).
Lixiong et al. (Stud. Surf. Sci. Catl.; 105 (1997), p 2211) reported synthesis of a SAPO-34 membrane on one side of a porous α-Al2O3 disk by immersing the substrate surface in a hydrogel and heating the substrate and gel. Lixiong et al. reported single gas permeances for H2, N2, CO2, and n-C4H10. Poshuta et al. (Ind. Eng. Chem. Res., 37 (1998), p. 3924-3929; and AlChE Journal, 46(4) (2000) p. 779-789) reported hydrothermal synthesis of SAPO-34 membranes on the inside surface of asymmetric, porous α-Al2O3 tubes. Poshuta et al. reported single gas and mixture permeances and ideal and mixture selectivities for several gases, including CO2 and CH4. The CO2/CH4 selectivities reported for a 50/50 CO2/CH4 mixture at 300K were between 14 and 36 for a feed pressure of 270 kPa and a pressure drop of 138 kPa (Poshusta et al, AlChE Journal, 46(4) (2000) pp 779-789). The CO2/CH4 selectivity was attributed to both competitive adsorption (at lower temperatures) and differences in diffusivity. Li et al. reported an average CO2/CH4 selectivity of 76+/−19 for a 50/50 CO2/CH4 mixture at 295 K with a feed pressure of 222 kPa and a pressure drop of 138 kPa. The average CO2 permeance was (2.3+/−0.2)×10−7 mol/(m2sPa) and the average CH4 permeance was (3.1+/−0.8)×10−9 mol/(m2sPa) (Li et al, Ind. Eng. Chem. Res. 44 (2005), 3220-3228) U.S. Pat. No. 7,316,727 to Falconer et al. reports CO2/CH4 separation selectivities of 67-93 for a 50/50 CO2/CH4 mixture at 297 K with a feed pressure of 222 kPa and a pressure drop of 138 kPa. Improved supported SAPO-34 membranes as well as methods for making them have further been disclosed in U.S. Pat. No. 7,828,875 (Li et al.); and U.S. Published Applications 2007-0265484-A1 (Li et al.) and 2008-0216650-A1 (Falconer et al.).
The SAPO-34 membrane permeance and selectivity can be affected by Si/Al ratios, seed size, template types, membrane thickness, cation forms, CO2/CH4 feed ratio and support properties (Li et al., Microporous Mesoporous Mater., 110 (2008) p. 310; Li et al. Adv. Mater., 18 (2006) p. 2601; Carreon et al., Adv. Mater., 20 (2006) p. 729; Hong et al., Microporous Mesoporous Mater., 106 (2007) p. 140; Li et al., J. Membr. Sci., 241 (2004) p. 121). Also, at high pressures both selectivity and permeance can be decreased by concentration polarization, although using a Teflon insert in a membrane tube has been used to minimize polarization (Avila et al., J. Membr. Sci., 335 (2009) p. 32).
Transport of gases through a crystalline molecular sieve membrane can also be influenced by any “non-zeolite pores” in the membrane structure. The contribution of non-zeolite pores to the flux of gas through a zeolite-type membrane depends on the number, size and selectivity of these pores. For polycrystalline molecular sieve membranes, some flow is expected through intercrystalline regions. If the non-zeolite pores are sufficiently large, transport through the membrane can occur through Knudsen diffusion or viscous flow. For MFI zeolite membranes, it has been reported that non-zeolite pores that allow viscous and Knudsen flow decrease the selectivity (Poshusta, J. C. et al., 1999, “Temperature and Pressure Effects on CO2 and CH4 permeation through MFI Zeolite membranes,” J. Membr. Sci., 160, 115).
Both permeance and selectivity for zeolite membranes can also be affected by the conditions used for organic template removal. Typically, zeolite membranes are prepared by contacting a membrane forming gel, which generally comprises Al2O3, P2O5, SiO2, H2O and an organic template, with a porous support and heating for several hours at temperatures in excess of 420 K. After hydrothermal synthesis, the membranes are typically washed, dried and calcined to remove the organic template.
In previous studies, the organic templating agent was removed from SAPO-34 membranes by heating the membrane in stagnant air (calcination) at 673 K with slow heating and cooling rates. Li et al. reported that the highest selectivities for SAPO-34 membranes were found for a calcination temperature of 663 K, and selectivities significantly decreased at temperatures above 673 K (J. Membr. Sci., 241 (2004) p. 121). Studies on MFI membranes reported that the thermal expansion mismatch between the zeolite film and the support can generate thermal stress and cracks in the membranes during template removal resulting in reduced performance (Gualtieri et al., J. Membr. Sci., 290 (2007) p. 95; Hedlund et al., J. Membr. Sci., 222 (2003) p. 163; Lai et al., Adv. Funct. Mater., 14 (2004) p. 716). In addition, intercrystalline pores may enlarge during template removal from the MFI structure because contraction of the zeolite unit cell causes tensile stress in the membrane layer (Dong et al., Microporous Mesoporous Mater., 34 (2000) p. 241). Woodcock et al. reported that chabazite, which has a similar structure as SAPO-34, was one of the most strongly contracting materials known, with a linear expansion coefficient varying from −0.5×10−6 to −16.7×10−6 K−1, and the unit cell volume decreased 1.5% from 293 to 873 K (Chem. Mater., 11 (1999) p. 2508). Therefore, the heating and cooling rates as well as the overall temperature during template removal affect pore size and the amount of defects in the resulting zeolite membrane and therefore can also affect the separation properties of the resulting membrane.
Although Gualtieri et al. concluded the residual stress in MFI films during calcination was independent of heating rate (Microporous Mesoporous Mater., 89 (2006) p. 1), the heating rate used during template removal has been shown to specifically affect membrane properties. Choi et al. showed that heating MFI membranes to 973 K in only 1 minute, prior to conventional calcination for 10 hours at 753 K (heating ramp of 0.5 K/min), significantly improved hydrocarbon separations at elevated temperatures (Science, 325 (2009) p. 590). They concluded that chemical bonds formed between the zeolite crystals during the rapid heating and these bonds minimized cracking during the slow calcination.
MFI membranes have also been prepared by template-free synthesis to minimize defect formation. Eliminating high-temperature calcination yielded membranes with higher separation selectivities (Pan and Lin, Microporous Mesoporous Mater., 43 (2001) p. 319; Hedlund et al., J. Membr. Sci., 159 (1999) p. 263; Gopalakrishnan et al., J. Membr. Sci., 274 (2006) p. 102; Zhong et al., Microporous Mesoporous Mater., 118 (2009) p. 224; Wegner et al., J. Membr. Sci., 158 (1999) p. 17). Hedlund et al. also reported lower permeances but higher H2/N2 ideal selectivities for ZSM-5 membranes prepared without a template (J. Membr. Sci., 159 (1999) p. 263).
An alternate approach to minimize defect formation in zeolite membranes is to remove templates at lower than usual temperatures. Ozone has been used in some studies to accomplish this, and Henga et al. reported complete template removal from 2-μm thick MFI membranes after 30 min at 473 K in oxygen that contained ozone (Kuhna et al., J. Membr. Sci., 339 (2009) p. 261; Motuzas et al., Microporous and Mesoporous Mater., 99 (2007) p. 197; Henga et al., J. Membr. Sci., 243 (2004) p. 69; Parikh et al., Microporous and Mesoporous Mater., 76 (2004) p. 17). Longer times were required for membranes that were thicker or had higher alumina content. In contrast, Kuhn et al. reported that fluxes for ozone-treated MFI membranes were 80% lower than those for membranes prepared by normal calcination, indicating that ozone did not completely remove the template (J. Membr. Sci., 339 (2009) p. 261). Motuzas et al. reported that ozone treatment, calcination at 0.2 K/min in air, and calcination at 5 K/min in air yielded MFI membranes with similar n/i-C4H10 ideal selectivities, but the permeances were lower for the ozone-treated membrane (Microporous and Mesoporous Mater., 99 (2007) p. 197).
Similarly, Parikh et al. removed templates from silicalite-1, AlPO-5, and ITQ-7 crystals in air at room temperature by using UV radiation from a medium-pressure mercury lamp (184-257 nm) to form ozone in-situ (Microporous and Mesoporous Mater., 76 (2004) p. 17). Templates have also been removed MFI and Beta type zeolites at 408 K by using solvents to decompose the templates (Jones et al., Micropor. Mesopor. Mater., 48 (2001) p. 57). None of these alternate methods for template removal were utilized for SAPO-34 membranes.
Kanazirev and Price (J. Catal., 161 (1996) p. 156) pointed out that previous studies indicate that the preferred procedure to activate many zeolite crystals, including BEA crystals, is thermal treatment in an oxygen-containing atmosphere with the final temperature being high enough to completely oxidize the template. However, they reported that a more efficient route to organic template removal is to first calcine the material in helium (750-800 K) so that the polymerization process which involves oxygen is suppressed, followed by a second higher temperature calcination step in oxygen (800-850 K). They observed, using TGA to measure weight loss, that oxygen caused the template to decompose/oxidize at a lower temperature than when the zeolite was heated in helium, but 33% of the template remained in the crystals that were heated in a 25% O2 stream (temperature less than 773 K). The residue from the partial oxidation was not removed until 900 K. They explained this un-expected behavior as due to formation of more stable species by polymerization, cyclization, and other reactions initiated by oxygen. They reported that the first calcination step under helium at approximately 750-800 K results in a material which contains a smaller amount of residue, which can then be removed through the second calcination step in the presence of oxygen at approximately 800-850 K.
Despite advances in this field, there remains a need in the art for improved methods of making zeolite membranes, in particular SAPO membranes, with desirable separation properties, such as high permeance and/or separation selectivities, and increased reproducibility.